This invention relates to high voltage apparatus in general, and more particularly to an improved construction for high voltage apparatus in which a conductor is enclosed by a housing and uses a gas under pressure as insulation inside the housing.
Apparatus of this general nature is known in which the high voltage apparatus with a voltage carrying part is enclosed by a housing or cladding with the housing supported by insulating members running between it and the voltage carrying part and which contains gas under pressure inside the housing which gas is used as insulation. In such devices, reinforcements are provided in the form of a double wall to prevent burn-off at the portions of the apparatus where standing arcs are expected in the case of trouble. An arrangement of this nature is shown for example, in U.S. Pat. application Ser. No. 201,464, filed Nov. 23, 1971, now U.S. Pat. No. 3,814,831.
High voltage networks are increasingly being equipped with completely housed switching installations which have compressed gas insulation not only at the nodal points, but also include housed tubular conductors having compressed gas insulation as opposed to the more common types of cables. Such is also provided for sections of line which are not constructed as open wire lines. These compressed gas insulated, completely housed tubular conductors are also referred to as compressed gas insulated high voltage tubular cables. At switching stations as well as in tubular conductors, single-and multi-phase housing is customary, which housing must meet certain safety requirements.
In the apparatus of the above mentioned U.S. Pat. application Ser. No. 201,464, filed Nov. 23, 1971, now Pat. No. 3,814, 831 a metal clad high voltage line is described, in which a closed, burn-off resistant ring is provided at the inside wall of the housing, adjacent to the insulating members separating the cable and its housing. At this portion there is thus what could be called an inner or inside wall and an outer or outside wall with the burn-off resistant ring being provided as the inside wall and the normal housing as the outside wall. A ring such as this ensures that an accidental occurring at the metal housing and which is unavoidably urged toward the nearest insulating member along the path of the current as a result of current forces will reach the burn-off resistant ring with its base starting at the housing. It can dwell, at that point, presuming suitable design of the ring, until interruption of the arc, thus, preventing destruction of the metal cladding or housing itself.
Arrangements of this nature are also described in an article by S. Fuhuda entitled "Current Carrying and Short Circuit Tests on EHV Cables Insulated with SF.sub.6 Gas" in IEEE Transactions on Power Apparatus Systems, vol. PAS-88 (1969), p. 147 to 153. As described therein, in tubular conductors the burn-off of the housing and the conductor due to accidental arcs is dependent on the material with which the cladding or the conductor is made. For example, an accidental arc will burn through a cladding of aluminum considerably faster than through steel cladding under otherwise equal conditions. This can be an advantage, because the sooner pressure relief occurs after the accident, the less probable is an additional pressure buildup and a major accumulation of hot gases and metal vapor resulting from accidental arcs. However, it is also a decisive disadvantage that aluminum cladding can burn through so easily at any point in that such can occur with short circuit currents of only a few kA, which currents have consequences which are essentially harmless. Thus, the prior art devices do not meet all the safety requirements required in an apparatus of this type.
Thus, it can be seen that there is a need for an improved arrangement of this type which avoids the problems present in the prior art.